Generally, many cellular and other communications systems operate in a frequency division duplex manner—simultaneous transmission and reception—using different frequency bands to transmit and receive, known as Frequency Division Duplex (FDD). As illustrated in FIG. 1A there is a frequency separation between transmit and receive bands. In a wireless transceiver, information is transmitted at a power level that is typically many times higher than the received power. Interfering energy, which is generated by the transmitter nonlinearities and other noise processes, is leaked into the receive band frequencies where it interferes with desired reception. This interference is referred to as self-interference in this description. Proper receiver operation requires attenuation of this self-interference, in many cases on the order of 100 dB or more.
Conventional mechanisms for attenuation of self-interference energy in the receive band of FDD systems include filter-based duplexers (air dielectric or ceramic filters) and diplexers, which are based on the ability to attenuate some frequencies while passing other frequencies due to the frequency separation between transmit and receive bands. For example FDD wireless base stations require a duplex filter to separate transmit and receive signals to/from a single antenna. Duplexers allow simultaneous transmit and receive operation through one antenna due to frequency-dependent filters between transmit and receive ports. The drawback is these components are typically quite large to meet the isolation requirements of modern communications systems. Isolation requirements in excess of 100 dB between the transmit power amplifier (PA) and the receive Low-Noise Amplifier (LNA) that share the same antenna. This requires the use of air-cavity filters roughly 10″×10″×2″ and larger. Multi-band operation of such systems requires complex and limited multiplexer designs or a very large switch network of duplexers. These duplexers are typically constructed of a metal housing with air-filled internal cavities and are therefore large and heavy. Multiple operating bands are also desired. This further exacerbates the linearity problem due to the difficulties in matching the PA for best linearity versus power over a wide bandwidth.
The continued worldwide adoption of 4G (e.g., Long-Term Evolution—LTE) adds other challenges, both in terms of the proliferation of bands and the increased interference emitted from the power amplifier. This increased interference requires larger duplexers. Further 4G operates over wider bands increasing the challenge of self-interference cancellation schema. In addition present art multi-band transceivers require the use of multiple duplexers which in present art are switched in and out for operation at a given band of operation of the base station. The trend for cellular waveforms is an increase in the requirement for transmit-to-receive isolation, which requires more difficult filter requirements. Another trend is the increase in number of defined bands. For example, LTE has nearly 40 defined bands. This can require a large number of filters for a multi-band transceiver. These problems are exacerbated when the transceiver operates at high power, for example, micro-class and macro-class base stations.
More recently, attempts have been made to remove self-interference in non-FDD frequency division duplex systems (where simultaneous transmission and reception occurs in overlapping frequencies) by use of electronic means or a combination of passive and active electronics means. Recent work done has focused on a solution for the case where the transmit and receive operations occur in the same frequency band. Heretofore, Time Domain Duplexing (TDD) architectures were employed in such cases, separating transmission and reception in time, rather than frequency. The present schemes provide an electronic means for attenuating the transmit signal at the receiver input. These schemes employ a balun-based or transformer-based coupler to provide to the receiver canceller a sample of the interference that is 180° out-of-phase and free from additional group delay. These schemes assume that the phase of the transmit signal seen by the receiver is constant across the band. In the disclosed non-full-duplex but FDD case, the phase of the transmit signal seen within the receive band is not constant and is a function of frequency, as a function of group delay −∂Φ/∂ω. Such frequency division duplex systems cannot operate for existing defined cellular standards or other wireless communication standards that separate transmit and receive energy based on frequency (FDD).
In radio frequency transmission systems which employ modulation schemes in which symbols contain both in-phase and quadrature-phase components, distortion exists between in-phase, I, and quadrature-phase, Q, components of a symbol constellation in the transmission between transmitter and receiver. In order to optimize the performance of the system in terms of improving error rate performance of the transmission channel this distortion is typically compensated for. Uncompensated for, distortion with respect to both phase and/or amplitude results in a received constellation in which the constellation is ether rotated in phase and/or offset in amplitude between I and Q components.
Frequency hopping for security of communications in multi-band systems has been slow and cumbersome, as reconfiguration of a transceiver between bands requires reconfiguration of duplexers and other elements of the system. Such reconfiguration has typically involved the use of relays. Providing security for radio communications via switching bands and operating frequencies during the operation of the system has been slow and cumbersome. Even more difficult is the operation of transmit and receive operations in different bands which may be separated by changing separation bands.